Hi
The 1881 census is free to search at
www.familysearch.orgThere are 3 David Ellis entries who were b Caenarvon in that age range but none specifically in Penmaenmawr. However - 2 of the entries have a second christian name which your David doesn't seem to have
This is a possible entry but as mum was a widow - it doesn't help much at the moment
Well Street Nevin, Caernarvon - all b Nevin
John JONES 73 Retired Mariner
Margaret 72
Fanny ELLIS widow 37 daughter Dress Maker
William 9 Grandson
David 6 Grandson
John 3 Grandson
Ref RG11 Piece 5561 Folio 62 Page 23
I'll check the 1871 for you